Most modern GPS receivers employ the GPS satellite almanac and rough information on current time and position to attempt to acquire signals of visible GPS satellites by searching in a limited number of frequency bins over a time uncertainty hypothesis of one millisecond, the repetition interval of the GPS C/A codes. In general, the entire sequence of events for arriving at a estimate of position location is in accordance with the following sequence of events:
1. Detection of a satellite PN code in a frequency bin, PA1 2. Acquisition and tracking of the carrier frequency, PA1 3. Acquisition and tracking of the data transitions and data frame boundary, PA1 4. Reading broadcast data for the satellite ephemeris and time model (the 900 bit Satellite Data Message), PA1 5. Completing steps 1-4 (serially or in parallel) for all in-view satellites, PA1 6. Making pseudorange measurements on these signals in parallel, and PA1 7. Computation of position using the pseudorange measurements and satellite data. PA1 Cold Start: Where the receiver has no GPS almanac. The GPS almanac is a 15,000 bit block of coarse ephemeris and time model data for the entire GPS constellation. Without an almanac, the GPS receiver must conduct the widest possible frequency search to acquire a satellite signal. In this case, signal acquisition can take several minutes to accomplish because a large number of frequency cells must be searched that takes into account the large uncertainties in satellite Doppler as well as GPS receiver oscillator offset. The terms "frequency bin" or "frequency cell" (used interchangeably herein) mean a narrow frequency range or spectrum, each frequency bin or cell having a characteristic center frequency and a predefined width or band of frequencies. In addition, acquisition of the GPS almanac will take at least 121/2 minutes of listening to the broadcast of a single GPS satellite. PA1 Warm Start: Where the receiver has a GPS almanac to aid the acquisition of satellite signals by greatly reducing the uncertainty in satellite Doppler and therefore number of frequency cells that must be searched. In this case, the number of frequency cells that must be searched is determined by the accuracy of the GPS local oscillator. For a typical oscillator accuracy of one ppm, the frequency search can be accomplished in less than 10 seconds. In this case, the major time bottleneck for generating a position fix is the time required to acquire the 900 bits of the Satellite Data Message for each GPS satellite that is to be used in computing the receiver position. This Message is broadcast every 30 seconds at 50 bps. For parallel GPS receiver channels, the time requirement to obtain the 900 bit Message from each in-view satellite is roughly 30 seconds. PA1 Hot Start: Where the receiver already has the Satellite Data Messages for all the in-view GPS satellites (7200 bits for eight satellites). In this case, the major time bottleneck is the acquisition of multiple satellite signals and generating pseudorange measurements from them (steps 6 and 7 above). The condition of a GPS receiver is "hot" if it recently (minutes) traversed the steps 1-5 above, or if it received the Satellite Data Messages from an alternate source. From a hot start, position determination begins at steps 6 and 7. This can be accomplished quite rapidly if a pseudorange measurement is utilized to calibrate out the frequency uncertainty of the GPS receiver oscillator, thereby enabling the rapid acquisition of subsequent satellite signals with a search over only a single frequency cell. Thus, from a hot start, it is possible to achieve a position fix very rapidly (in less than one second) if a search algorithm is used that minimizes the required frequency search band for signal acquisition.
The time required to accomplish these steps in a conventional GPS receiver will vary depending upon the assumed starting point of the GPS receiver. It is useful to define three reference starting points for a GPS receiver. These are as follows:
This invention merges GPS position location and wireless data communication technologies to achieve a precise position location via GPS in the urban canyon and other line-of-sight obstructed environments. A multi-channel GPS receiver with the capability to simultaneously track (and make pseudorange measurements with) all in-view GPS satellites is used in conjunction with an algorithm that makes maximum use of all a prior information about the GPS receiver (its oscillator bias, its location, its knowledge of time) and the ephemeris and time models of the GPS constellation received by a wireless data communication channel or link to enable rapid acquisition of the GPS signal.
As shown above, currently, there are two time bottlenecks in estimating accurate position via GPS. One of these is due to the oscillator bias of the GPS receiver which is a driver for a time consuming search over many frequency cells.
According to the invention, the search over frequency is required only for the acquisition of the first GPS satellite. The frequency measurement from tracking that one satellite is then used to calibrate out the frequency bias of the GPS local oscillator. Thus, the subsequent acquisition of other GPS satellite signals can be accomplished very rapidly because the number of frequency cells that must be searched is reduced to one.
The second time bottleneck in determining precise position location is the necessity to read the 900 bit GPS Satellite Data Message block containing the ephemeris and satellite clock models of the GPS satellites. This data message must be extracted for each satellite that is used for the GPS position solution. Extracting this needed information for determining position will take 30 seconds in a clear environment; in an obstructed environment, extracting this information may take far longer, and in the worst case, may not be possible at all. According to the invention, this is supplied to the GPS receiver with the needed ephemeris and satellite clock information via an independent wireless data channel such as can be supported by an RDS FM broadcast or a cellular telephone channel. With a cellular telephone, the needed data can be supplied by calling (or receiving a call from) a service center and establishing a data link via a modem in the cellular phone, and a modem to a service center. The required GPS satellite information is then supplied via the established data link. At typical modem speeds (1.2 Kbps to 19.6 Kbps), this information is supplied in only a few seconds to less than one second, depending upon the modem speed. In this manner, the GPS is assisted in rapid signal acquisition and rapid determination of position, even in obstructed environments.